A conventional backhoe includes an articulated boom mounted on the rear of a tractor or similar piece of equipment which carries a pivotal bucket for digging operations. The boom is mounted to a swing tower for movement about a vertical axis so that material carried by the backhoe bucket may be moved from one area to another. The swing tower is rotated from side to side by opposed double acting hydraulic motors controlled by a directional control valve manipulated by the backhoe operator.
Backhoes are employed for a wide variety of material handling and excavation operations, and as a result the business is highly competitive in nature. In view of this, any means whereby the work can be more efficiently performed is desirable. One of the ways in which efficiency may be increased is to shorten the time cycle involved in filling the bucket, raising it out of the excavation, swinging the bucket laterally, depositing the material within the bucket on a pile or into a truck, and then returning the bucket to repeat the cycle.
With conventional hydraulic arrangements employed prior to the 1960's for rotating the swing tower of the backhoe, it was the usual practice of operators, in order to save time, to swing the boom and swing tower over hard against the mechanical travel or swing stops provided on the backhoe frame for limiting the arc of swinging movement. This practice was found to be very detrimental because the frame, the swing tower and boom, and the hydraulic circuits were subjected to severe shock loading. While these shocks could be minimized by careful manipulation of the backhoe swing controls, this extra degree of care proved to be time consuming, and thus decreased productivity.
Thus, in order to alleviate this problem while improving the productivity and efficiency of the backhoe, various systems have been devised to decelerate the boom and swing tower prior to hitting the swing stops, even if the backhoe operator does not attempt to reduce the speed of the boom.
One prior method of cushioning movement of the boom and swing tower as they approach the stops at the end of the arc of travel includes substantially blocking the normal flow port from the cylinder end of each of the hydraulic motors to restrict fluid flow. Flow is blocked by a projection carried by the piston of each of the hydraulic motors that enters and substantially blocks flow in a motor outlet port as the piston moves within the motor. Projections such as these are sometimes referred to as "stingers." Although such arrangements are still commonly in use today, their fabrication and maintenance has proven to be relatively expensive.
Another arrangement for providing cushioning for the movement of the boom and swing tower is to include an orifice in the outlet port of the hydraulic motors. In this way, back pressure is created within the hydraulic motors which acts to resist the continued swinging movement of the boom and swing tower. This arrangement is not without its drawbacks, however. The pressure generated by the orifice is continually resisting the swinging movement of the boom and swing tower, even when the backhoe operator is trying to accelerate the swinging movement. This acts to lower the speed of the swinging movement, uses more energy than is necessary to swing the boom, and generates more heat in the hydraulic system. Further, the use of such orifices does nothing to slow or cushion the swing of the boom and swing tower toward the extreme ends of the arc of travel because the fluid flow through the orifices is too small to generate sufficient pressure to slow the swinging movement. In view of this, use of orifices in combination with the above-described stingers is not uncommon, but such arrangements are fairly expensive and may be subject to problems during field use.
Another area of backhoe swing mechanism design which has created problems relates to the positioning and hydraulic porting of the hydraulic motors. Part of the versatility of backhoes is derived from their ability to rotate the swing tower and boom through an arc of approximately 180 degrees. Although various arrangements have been tried, spacial limitations have generally required that the hydraulic motors be mounted on the backhoe frame generally parallel to each other and on respective sides of the vertical axis of the swing tower. It will be appreciated, however, that this arrangement creates problems when the swing tower is rotated through the desired arc of travel.
Specifically, as the swing tower and boom rotate in one direction or another, from a centrally disposed position, one of the hydraulic motors extends to a fully extended condition which occurs as the centerline of that motor intersects the vertical axis of the swing tower. When this occurs, the motor is frequently referred to as being in its "center" position. As the swing tower continues to rotate toward the travel stop, that motor would start to contract, and would be referred to as being in an "overcenter" position or condition. If the supply of pressurized hydraulic fluid to the hydraulic motors is continued and ported without change as one of the motors goes overcenter, the pressure of the fluid causes that motor to exert a "negative torque" on the swing tower and boom. Becaue of the geometry of the swing tower and the hydraulic motors, the hydraulic motor which has gone overcenter acts upon the swing tower through a lesser moment arm than the other hydraulic motor of the swing mechanism. Consequently, the swing tower continues to move as intended, with the one motor not only rotating the swing tower and boom, but working to overcome the negative torque created by the overcenter hydraulic motor. Thus, a swing mechanism control system which operates to eliminate the negative torque created by one of the hydraulic motors in an overcenter configuration as the swing tower and boom are moved provides a more efficient swing mechanism system.
It is particularly desirable to eliminate this negative torque exerted by the overcenter motor as the swing tower and boom are moved away from their travel stop. This improves the net torque applied to the boom assembly. Further benefit is desired if the overcenter motor can be ported to provide a supplemental torque to the boom assembly which assists the motor providing the primary torque in initiating swinging movement of the mechanism.
Thus, a hydraulic valving arrangement for a swinging mechanism of a backhoe which acts not only to alleviate the problems of cushioning the boom and swing tower assembly, but also improves the operational characteristics of the assembly, particularly toward the ends of its arc of travel (when one of the hydraulic motors is in an overcenter position), would be extremely desirable.